Null and spectroscopic ellipsometer systems for use in investigation and characterization of physical and optical properties of substrate systems are well known. Briefly, such systems operate by monitoring changes effected in the polarization state of a beam of light when said beam of light is caused to interact with a substrate system. Spectroscopic ellipsometer systems, including those which utilize phase modulation and Rotating Analyzers, (ie. rotating analyzer ellipsometer systems, hereinafter (RAE)), are widely applied because they provide improved theoretical precision and high optical efficiency, hence, can be utilized with weaker polychromatic sources of light as compared to null ellipsometer systems. Spectroscopic ellipsometer systems are also faster and easier to use. However, use of spectroscopic ellipsometer systems requires increased attention to compensation necessitated by, for instance, Polarization-Dependent Sensitivity, (hereinafter, (PDS)) in response to applied polychromatic light. Compensation of (PDS) can be approached in two mathematically oriented ways, one of which requires correction of numerous raw data points obtained during investigation of a substrate system by a spectroscopic ellipsometer system, (eg. an RAE), and the second of which applies correction factors to, for instance, Fourier Coefficients, (termed ALPHA and BETA), derived by application of Fourier Analysis to said numerous raw data points. Of said approaches, the second is generally easier to perform and is preferred.
To understand the second approach to mathematically compensating (PDS) it must be appreciated that the end goal of applying ellipsometry to a substrate system is simultaneous characterization of the physical and optical properties of said substrate system. Two ellipsometric constant, (at a particular light beam wavelength and angle of incidence on said substrate), parameters, PSI and DELTA serve to provide said characterization. Calculation of PSI and DELTA, however, is typically intermediated by the calculation of Fourier Coefficients, ALPHA and BETA, as alluded to above. Said Fourier Coefficients, ALPHA and BETA, are related to PSI and DELTA by known mathematical relationships. Briefly, to compensate for system (PDS), ALPHA and BETA can be corrected prior to application of the mathematical relationships which interrelate the Fourier Coefficients ALPHA and BETA, to PSI and DELTA. It is noted that ALPHA and BETA correction factors are typically derived during an ellipsometer system calibration procedure. It is mentioned that direct correction of PSI and DELTA is a variation on said mathematical approach.
As mentioned, PSI and DELTA are constants of a substrate system, but as also mentioned, said constants vary with the wavelength of a beam of light applied to a substrate system by an ellipsometer system. This is because ellipsometer system elements, (eg. dispersive optics), as well as investigated substrate systems, typically respond differently to different wavelengths of light. (Note, see the Disclosure of the Invention and Detailed Description Sections in this Disclosure for insight as to the elements, and their configuration, which comprise a typical ellipsometry system). As a result, an ellipsometer system which performs a substrate system analysis at a multiplicity of wavelengths must provide appropriate ALPHA and BETA (PDS) correction factors for each of said multiplicity of wavelengths utilized, (assuming the second approach to compensating (PDS) identified above is utilized).
Continuing, it is to be appreciated that while compensation can be accomplished based upon a purely mathematical approach, correction factors can become a significant percentage of an ALPHA or BETA value at certain corresponding PSI and DELTA values. When this occurs, application of a correction factor can result in relatively small corrected ALPHA and/or BETA values, which values can be on the order of system background noise. This results in reduced sensitivity, (ie. reduced ability to calculate accurate PSI and DELTA values from said corrected ALPHA and BETA values), at said certain corresponding PSI and DELTA values. It should then be apparent that a system element which would reduce the magnitude of required ALPHA and BETA correction factors would provide utility. A similar situation exists when PSI and DELTA are directly corrected.
It is also mentioned that (RAE's) often comprise elements, (eg. photodetectors), which react nonlinearly with respect to different intensities and wavelengths of light. The above outlined mathematical approach to compensation can be used in such (RAE's) to simultaneously compensate both (PDS) and said nonlinearities.
Continuing, the present invention teaches that a (RAE) in which a diffraction grating comprises a dispersive optics system element, which diffraction grating is situated prior to a detector element and serves to provide a multiplicity of independently detectable light wavelength beams to said detector element, should have (PDS) response wavelength dependence reduced by application of a specific system element to said (RAE), and by practice of a specific method of use thereof. The present invention method of use includes the teaching that compensation of said (RAE) (PDS) can be further effected by mathematical application of ALPHA and BETA correction factors, or by direct mathematical correction of PSI and DELTA values.
A search of relevant references has provided an article by Russev, App. Optics, Vol. 28, No. 8, April 1989, p. 1504-1507. An approach to mathematically calculating ALPHA and BETA correction factors to compensate for (PDS) and/or detecting system nonlinearity, either independently or simultaneously, is described in said reference. As well, said reference mentions the use of a depolarizer between a rotating analyzer and a detector as a means of reducing (PDS), but notes that said approach is not complete because of residual beam polarization. In addition, the presence of a depolarizer is stated to be undesirable because it reduces light flux reaching the detector.
In view of the approach described in the Russev reference, it is noted that an article by Johs, Thin Solid Films, 234(1993), p. 395-398, describes an improved approach to determining and applying ALPHA and BETA correction factors using a regression data fitting approach over a large range of polarizer azimuth angles.
The Russev and Johs articles cited above are incorporated by reference into this Disclosure.
A U.S. Pat. No. 4,837,603 to Hayashi, describes a method of correcting azimuth angle of photometric ellipsometers by an approach which mathematically corrects PSI and DELTA. Said reference is also incorporated by reference into this Disclosure.
An article by Stobie et al., J. Opt. Soc. Am. 65, p. 25 (1975), describes application of a double modulation which could be used to circumvent (PDS) of dispersive optics. The system requires that a polarizer, (ie. polarization state generator), and an analyzer (ie. polarization state detector), be set at known azimuths, and that a rotating analyzer, (ie. modulator), be present before or after a sample. As it is difficult, however, to determine azimuths of a polarizer and analyzer, their being nonlinear functions of ellipsometric measurement parameters, use of the ellipsometer system described in said reference is relatively complex and in some applications unsuitable for application to spectroscopic ellipsometer systems because sensitivity to certain values of PSI and DELTA is reduced. It is noted that the Stobie et al. article fails to suggest that the system described therein should be used to compensate (PDS).
An article by Collins, Rev. Scien. Inst., 61(8), August 1990, p. 2029-2062, describes an ellipsometer system which uses a rotating polarizer. Said system could be used to remove (PDS) of dispersive optics but introduces (PDS) to the system light source. In said system an analyzer is set at a known azimuth and modulation is introduced in the polarization state generator. Said configuration requires calibration of residual light source polarization, and light beam precession on the sample can occur during use. This is especially unsuitable wherein monitoring of a real time, in situ process is involved. As well, the Collins article fails to suggest use of the system described therein to compensate (PDS).
The above discussion should serve to demonstrate that a system, and method of use thereof, which can easily, simply and efficiently serve to reduce (PDS), and which does not impose any unnecessary restraints on (PDS) reducing system element design or configuration, would provide utility. Such a system and method of use are taught by the present invention.